Zone melting



Jan. 21', 1969 w. e. PFANN ZONE MELTING Fi led Jan. 15, 1966 2052pmuqmtbm m M20 050: #243000 W450 QZEQMI lNl/ENTOR W G. PFA NN ATTORNEYUnited States Patent 7 Claims ABSTRACT OF THE DISCLOSURE Thespecification describes a zone refining apparatus in which the zones aremoved longitudinally through a horizontally disposed continuouscylindrical charge. Zone dimensions are maintained uniform by rotatingthe cylinder and are minimized by forced cooling.

This invention relates to zone melting.

A typical zone melting process is a multipass operation and it is oftenadvantageous to provide as many molten zones as possible within a givenlength of material to minimize the size of the apparatus and theprocessing time. This goal establishes a small zone length as a generalobjective in zone refining methods.

Zone melting processes which are carried out in an essentially closedtube would ordinarily operate with the tube disposed vertically. Theunfortunate consequence of this arrangement is that heat convectioncontributes directly to a lengthening of the molten zones. It may betheorized that the adverse effects of heat convection may be minimizedby disposing the tube horizontally and passing the zones along itshorizontal axis. However, when the tube is placed horizontally, theeffect of heat convection is to distort the zone boundaries such thatthe upper region of the zone is considerably longer than the lowerregion. Planar and parallel zone boundaries are desirable for variousknown reasons but especially for providing a large number of zones in asmall length.

According to one aspect of the present invention the zone length isdramatically reduced by the combined expedients of disposing the tubehorizontally and passing the zones along the horizontal axis, coupledwith a slow rotation of the tube about its own axis. Slow rotation isintended as meaning from about one-half to twenty-five r.p.m.

As suggested in the foregoing discussion, the horizontal position of thetube largely restricts the direction of natural convection currents tothe plane of the zone. If the tube is stationary, the convectioncurrents, confined in the upper regions of the tube, begin to spreadhorizontally which results in a widening of the liquid zone. Theconvection flow pattern is of course due to variations in the density ofthe liquid. The major driving force for the convection flow is gravity.When the tube is rotated slowly the gravitational influence is alteredand the characteristic convection flow pattern which produces thehorizontal spread at the top of the tube is destroyed. The convectionflow pattern remaining with the tube rotating is likely to be angular.Intuitively, it will be appreciated that an anguler flow pattern in theplane of the zone contributes to uniform zone boundaries.

It will often be found advantageous to forcibly cool the solid regionsbetween the zones. This results in a further decrease in the zone lengthand in many cases will be found essential to a practical refiningoperation. This is especially true of low melting solids, particularlythose with only moderate heat conductivities. Accordingly, a preferredform of this invention involves the use of alternate heating and coolingmeans disposed around a rotating horizontal tube. As is well known, thezones may be "ice passed by moving either the heating means or the tube.Usually the latter is more convenient in the multizone operation of thisinvention. Any known cooling means may be employed. However, as anotheraspect of this invention a cooling means is employed which is especiallyeflective for the desired purpose. One outstanding feature of thiscooling means is that it flows the cooling fluid in direct contact withthe tube itself and confines it to a precise flow path; yet it is notphysically attached to the tube and the tube can move freely through thecooling as well as the heating zones while the cooling and heatingapparatus is held stationary.

These and other aspects of the invention will become more apparent uponconsideration of the following detailed description. In the drawing:

FIG. 1 is a front view, in perspective, of a multipass zone meltingapparatus constructed according to the principles of the invention; and

FIG. 2 is a perspective view partly in section illustrating theoperation of the novel cooling apparatus.

In the apparatus of FIG. 1 the tube 10, containing the material to berefined, is carried by a support jig (not shown) for supporting the tubein a horizontal position and slowly moving the tube back and forth inthe direction indicated by the arrows. The support jig is also designedto rotate the tube slowly about its axis. The heating wires used to formthe molten zones are indicated at 11 and consist here of several similarNichrome wire rings disposed around the tube 10 and spaced therefrom adistance which permits both adequate heating and free movement of thetube. The distance is not critical and the rotation of the tube,according to an essential feature of the invention, allows forirregularities in the spacing of the wire from the tube and in thecircumferential uniformity of the heating sources which might otherwisecreate difliculties. Various alternative heating means may be employedsuch as U-shaped heaters although ring heaters have been found to beparticularly effective.

The cooling means are a series of rings 12 connected to hollow inlettubes 1'3 at the top and outlet tubes 14 at the bottom. The rings may beof any appropriate material but preferably consist of a metal with goodheat conductivity, such as copper, silver, gold, molybdenum, or an alloysuch as brass. It is important that the ring have a significant axialdimension for reasons which will become apparent.

The operation of the cooling means is illustrated in FIG. 2 and forms adistinct part of the invention.

FIG. 2 is a perspective view with the tube 10 and its contents 15 shownin section. The coolant flows in the tube 13 at the top, convenientlywith a gravity feed. The fluid flows around the tube 10 through theannular space 16 between the tube and the cooling ring 12. The charge 15is maintained essentially solid in the vicinity of the cooling ring. Thefluid exits through outlet tube 14. The outlet tube is not essential butdoes contribute to the effectiveness of the apparatus by producing ahydrodynamic head on the fluid in the annular space 16 and actuallyincreases the fluid velocity. It also aids in confining the coolant inthe annular space 16 due to the siphoning effect and provides aconvenient means for collecting the coolant at the several ring coolers.

The annular space 16 is not closed as is evident from FIG. 2 sinceproviding a seal to the tube 10 which permits the tube to rotate is adifficult problem. However, with the arrangement shown, surface tensionconfines the coolant to the annular space 16 and no liquid seal isnecessary. Part of the heat removed from the charge is carried off bythe coolant and part is removed by conduction to the metal ring andattached tubes. It is important that the inner surface be wetted by thecoolant which usually reqiures the ring to be thoroughly clean.

The effect of using alternate heating and cooling means according tothis particular embodiment is shown in FIG. 1. The liquid zones areshown at 17 separated by solid regions 18.

The following specific example is given to illustrate the effectivenessof the invention.

The tube 10 was a 2.54 cm. O.D. Pyrex tube approximately 50 cm. long.The heating rings were one turn of 0.5 mm. diameter Nichrome wire. Theheaters were connected in series and powered by 60 c.p.s. current. Acurrent of 4 or 5 amperes is approximately adequate to produce adesirable molten zone in the apparatus illustrated. Minor adjustmentscan be made for each heater by providing a variable shunt resistoracross each heater. Once the heaters are set the apparatus can run fordays at a time without further control of the heaters. The heating ringswere spaced 2.5 cm. apart and a total of heaters were used. Elevencoolers are provided as shown in FIG. 1. The cooling elements 12 (FIG.2) are copper rings 2.60 cm. I.D. having an axial width of 0.6 cm. inlet(13) and outlet 14 tubes are connected to the ring as shown in FIG. 2.The inlet and outlet tubes are copper having a 0.5 cm. diameter. Thecoolant used was water with the flow rate adjusted to about 70 cmfi/min.

The charge material was naphthalene which orginally was a yellowishwhite showing obvious contamination. The mean zone length was 0.6 cm.,roughly one-quarter of the tube diameter. The tube was moved axially ata rate of approximately 0.8 cm./hr. and rotated at 1.3 r.p.m. In thisapparatus the heaters and coolers are stationary. Obviously the reversearrangement can be used. After the passage of several zones, thematerial at the front end of the tube becomes whiter in color andobviously purer than the original charge.

In using this rotating tube technique voids may appear in the zone. Forexample, if the initial solid charge is sufficiently porous, and amolten zone is formed in it, the liquid may not occupy the entire crosssection of the tube, even allowing for the increase in volume onmelting. A void, or bubble, appears at the top of the zone. As the zoneadvances, the void moves with it, and a nonporous solid is formed behindthe zone. If, however, the void occupies more than half the crosssection, a hollow pipe will appear behind the zone along the axis of thecharge.

Another way that a void can be formed is contraction of an establishedmolten zone that occupies the entire cross section of the tube.Contraction produces solid, which occupies less volume than the liquidfrozen to produce it, and hence a void appears.

In either case, the void is carried to the end -of the charge, and anonporous solid is produced behind the zone. Movement of the void to theend of the charge corresponds, of course, to transport of matter towardthe beginning of the charge.

The formation of nonporous solid behind the zone in the convectiveheating technique exacerbates the problem of tube breakage (formaterials that increase in volume on melting). If the front end of thecontainer is unyielding and is completely filled with nonporous solid,the volume increase when a zone enters the charge is likely to crack thecontainer. Such tube breakage can be avoided in several ways.

The simplest way, when feasible, is to place a slidable silicone rubberplug (or other inert material) at each end of the charge in an otherwiseopen tube. Repeated zone passes move the plug at the beginning of thecharge backwardthat is, opposite to the direction of zone travel-- andproduce a larger and larger void at the end of the charge. The void canbe removed by pushing in the end plug when the end of the charge isliquid.

This backward migration of the charge can be appreciable. For example,passing 27 zones averaging 0.9 cm. long through a charge of naphthalenemoved the front plug about 4.5 cm. (which corresponds well with thedensity change on melting). The migration gradually moves the beginningof the charge out of the array of reciprocating heaters. This might bedesirable from a purification point of view, but it is undesirable inthat starting a zone some distance from the front end of the charge maycrack the tube, for reasons mentioned. We have usually chosen to movethe heater array with the charge by changing the positions of the limitswitches that control the reciprocating motion.

Another way to prevent cracking, which obviates use of a sliding plug,is to seal a glass partition in the tube at the location of thebeginning of the charge. A small hole is left in the partition near itsperiphery. When a zone forms at the beginning of the charge, expansionforces liquid out of the zone through the hole. Once the pressure isrelieved, no more liquid flows out, because of the difliculty ofnucleating a void in the zone. This method also produces a larger andlarger void at the end of the charge, which can either be left there, orreplaced with fresh charge material as dsecribed below.

A third way to prevent cracking is to leave an unmelted plug of solid ateither end of the charge, with no other restraint. If the solid isdeformable, or if it does not stick to the tube, it can behave just asthe silicone plug described above. Again a void will grow at the end ofthe charge. If the plug of solid at the end of the charge is long, onecan melt some of this solid and, by tilting the apparatus briefly, letthe void bubble out, and at the same time let fresh charge material mixwith the impure material in the last zone length.

If the substance of the charge contracts on melting the cracking problemshould be less serious. The expected behavior, for a charge tubeinitially filled wit-h nonporous solid, is formation of a void when thezone enters the charge, travel of the void with the zone to the end ofthe charge, and disappearance of the void as the zone leaves the charge.

The translation rate of the zone must be tailored to the effectivenessof the heat sink and the natural convection in the zone. If the rate isincreased beyond the proper value, the zone lags behind the heater, themelting interface becomes convex toward the liquid, and the freezinginterface becomes concave toward the liquid. For 2.5 cm. tubes, and fora number of organic compounds, about 2 cm./hr. has been found to be asuitable maximum rate with respect to control of zone shape and zonesize.

The rotation rate should be great enough to prevent the tapering efiectdiscussed earlier for a stationary horizontal tube yet not so great asto seriously alter the flow pattern due to natural convection. Rates offrom 0.5 r.p.m. to about 25 r.p.m. have been found suitable for tubes ofa few centimeters in diameter. If the rotation rate substarrtiallyexceeds these values in many cases control over the zone shape anddimensions is lost.

These parameters are exemplary and particularly adapted to materialshaving relatively low thermal conductivity. The invention in a preferredform is applied to materials having a thermal conductivity of less than0.01 cal/cm. sec. C. The specific values of the various parameters for adifferent apparatus and charge material may best be determinedempirically. In refining higher melting materials a forced coolingarrangement may be unnecessary. This will also be the case for refininglower melting materials if a wider zone spacing can be tolerated.

Whereas this discussion has been presented largely in terms of zonerefining the invention is obviously applicable to other relatedprocesses such as zone leveling, normal freezing and the growth oflarge-diameter single crystals. The term zone melting is used herein inits generic sense.

Various additional modifications and extensions of this invention willbecome apparent to those skilled in the art. All such variations anddeviations which basically rely on the teachings through which thisinvention has advanced the art are properly considered within the spiritand scope of this invention.

What is claimed is:

1. A zone melting apparatus comprising a hollow cylindrical elongatedlinear tube having an axis along its length, for containing the materialto be refined, means for mounting the tube in a horizontal position, atleast one heating element, said element surrounding a substantialportion of the circumference of the tube and spaced therefrom to permitrelative axial movement between the tube and said heating element, meansfor causing relative axial movement between the tube and said heatingelement and means for rotating the tube around the axis.

2. A zone melting method which comprises circumferentially heating anelongated linear horizontally disposed tube containing a material to bemelted to develop a molten zone in said material while continuouslyrotating the tube about its axis and passing the molten zone along theaxis of the tube.

3. The method of claim 2 wherein the tube is rotated at a speed of from0.5 to 25 revolutions per minute.

4. The method of claim 2 wherein the material being refined has athermal conductivity of less than 0.01 cal./ cm. sec. C.

5. A zone melting apparatus comprising a horizontally disposed elongatedtube having an axis along its length for containing the material to bemelted, means for passing a molten zone axially along the tube, coolingmeans arranged to cool at least one of the zones adjacent the moltenzone, said cooling means comprising a cylindrical ring encircling butspaced from the tube and having an axial dimension which issubstantially less than the tube diameter, and means for introducing aliquid coolant into the upper region of the space between the tube andthe cooling ring so that the cooling liquid will flow by gravity aroundthe periphery of the tube, the space between the tube and the ring beingsuch that the cooling liquid is confined to the space between the tubeand the ring by surface tension.

6. The apparatus of claim 1 wherein a cooling means is provided adjacentto the heating means.

7. The apparatus of claim 6 wherein the cooling means is a liquidcoolant.

References Cited UNITED STATES PATENTS 2,823,102 2/ 1958 Selken 23-2732,967,095 1/ 1961 Herrick 23-273 3,124,633 3/1964 Van Run 23-3013,139,653 7/1964 Orem 23-273 3,189,419 6/1965 Wilcox 23-301 3,258,314 6/1966 Redmond 23-301 2,998,335 8/1961 De Hmelt 23-301 FOREIGN PATENTS191,165 9/1964 Sweden.

OTHER REFERENCES Pfann Zone Melting (pp. 62-71), 1958. Wiley and Sons.

NORMAN YUDKOFF, Primary Examiner.

G. P. HINES, Assistant Examiner.

US. Cl. X.R. 23-273

